The present invention relates to a superconductor; and, more particularly, to a superconductor incorporating therein an intermetallic compound superconductivity epitaxial thin film and a method for manufacturing the same, using a superconductor material such as magnesium diboride (MgB2) or (Mg1-xMx)B2.
In general, a fabrication of a superconductivity thin film has been advanced for tens of years for the purpose of an electronic circuit application. Particularly, the fabrication of the thin film and its application to the electronic circuit has been mainly researched and developed by utilizing niobium (Nb) of a low temperature superconductor and Y1Ba2Cu3O7xe2x88x92x (YBCO) of a high temperature superconductor, wherein a superconductivity transition temperature (Tc) of Nb is 9.2 K and that of the YBCO is 93 K.
The superconductivity transition temperature (Tc) of a YBCO thin film is higher than Tc, i.e., 77 K, of liquid nitrogen, and an energy gap of the YBCO thin film is greater than that of the low temperature superconductor so that it may be applied to the electronic circuit with a high speed performance. But, the YBCO thin film has a limitation that a uniform junction is too hard in a manufacturing process so that it is difficult to manufacture an integrated circuit (I.C).
On the contrary, Nb of the low temperature superconductor has advantages that the junction process is easy so that it may be applied to the fabrication of the I.C. However, an operation of the I.C is performed at a temperature below than the superconductivity transition temperature of liquid helium (He), i.e., about from 4 K to 5 K, so that the Nb thin film is less practical in the view point of economy.
In recent years, magnesium diboride (MgB2) is discovered as the superconductor material. The MgB2 is an intermetallic compound superconductor having magnesium (Mg) and boron (B) therein. The composition of the MgB2 is relatively simple and the superconductivity transition temperature is high, i.e., 39 K, so that it can be applied to the electronic circuit in case of fabricating MgB2 as the thin film.
If MgB2 is applied to the electronic circuit, the operation of the circuit can be performed at the temperature ranging from 15 K to 20 K using a conventional cryocooler and the speed of the circuit is approximately 4 times as fast as that of an Nb circuit which is operated at the temperature ranging from 4 K to 5 K. In addition, since an operation temperature of the circuit ranges from 15 K to 20 K, it is unnecessary to use liquid nitrogen so that it may be widely applied to an electronic device economically.
Up to now, several technologies for MgB2 has been announced such as a fabrication technology of an MgB2 powder, an MgB2 pellet and an MgB2 wire.
Referring to FIGS. 1 and 2 are graphs setting forth a X-ray diffraction pattern and a relation between the resistivity and the temperature of an MgB2 thin film in accordance with a first prior art. According to the first prior art, the method for manufacturing the MgB2 pellet begins with mixing Mg and B to a ratio of 1:2. Thereafter, mixture of Mg and B is pressurized at a high temperature in a hot isostatic pressing (HIP) furnace, thereby obtaining an MgB2 pellet having the superconductivity transition temperature of 39 K. This is disclosed by J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu in an article, xe2x80x9cSuperconductivity at 39 K in Magnesium Diboride, Nature 410, 63, 2001xe2x80x9d.
A second prior art for manufacturing the MgB2 pellet is disclosed by C. U. Jung et al., in an article, xe2x80x9cTemperature-and-Magnetic-Field-Dependences of Normal State Resistivity of MgB2 Prepared at High Temperature and High Pressure Condition, http://www.lanl.gov/cond-mat/0102215xe2x80x9d. In a disclosure, MgB2 is fabricated in a type of pellet under the high temperature and the high pressure by using an anvil-typed press.
Furthermore, a third prior art for manufacturing the MgB2 powder is disclosed by S. L. Bud""ko et al., in an article, xe2x80x9cBoron Isotope Effect in Supercoducting MgB2, Phys. Rev. Lett., 86, pp. 1,877-1,880, 2001xe2x80x9d. According to the third prior art, to begin with, mixture of Mg and B is inserted into a tantalum (Ta) tube after Mg and B are mixed to a ratio of 1:2. Thereafter, the Ta tube is vacuum-sealed using a quartz capsule. Finally, the Ta tube provided with mixture of Mg and B therein is annealed at 950xc2x0 C. and then cooled, thereby obtaining the MgB2 powder.
A fourth prior art for manufacturing the MgB2 wire is disclosed by P. C. Canfield et al., in an article, xe2x80x9cSuperconductivity in Dense MgB2 wire, Phys. Rev. Lett., 86, pp. 2,423-2,426, 2001xe2x80x9d. In a paper, boron fiber and Mg are inserted into the Ta tube and the tube is vacuum-sealed using the quartz capsule. Thereafter, it is annealed at 950xc2x0 C., thereby obtaining the MgB2 wire.
However, it is impossible for the MgB2 powder and pellet and wire fabricated by the prior arts to be applied to the fabrication of the electronic circuit. Thus, a fabrication method for the MgB2 thin film is required to be applied to the electronic circuit.
It is, therefore, an object of the present invention to provide a superconductor incorporating therein a superconductivity epitaxial thin film of magnesium diboride (MgB2) or (Mg1-xMx)B2 which can be applied to a rapid single flux quantum (RSFQ) circuit.
It is, therefore, another object of the present invention to provide a method for manufacturing a superconductor incorporating therein a superconductivity epitaxial thin film of MgB2 or (Mg1-xMx)B2 which can be applied to a rapid single flux quantum (RSFQ) circuit.
In accordance with one aspect of the present invention, there is provided a superconductor comprising: a template film having a hexagonal crystal structure; and a superconductivity thin film formed on top of the template film, including magnesium (Mg) and boron (B) therein which are epitaxially grown up, wherein a crystal structure and a lattice constant of the template film are similar to those of the superconductivity thin film.
In accordance with another aspect of the present invention, there is provided a method for manufacturing a superconductor comprising the steps of: preparing a substrate; b) forming a template film on top of the substrate, wherein the template film has a hexagonal crystal structure; and c) forming a superconductivity thin film on top of the template film having Mg and B therein.